1. Field of the Invention
The present invention relates to an electrophotographic photoconductor which has a high sensitivity and excellent durability, and is capable of producing stable images without deterioration of the image quality under use for a long period of time, and a process cartridge and electrophotographic apparatus using the same.
2. Description of the Related Art
The electrophotographic method which utilize an electrophotographic photoconductor has been applied to copying machine, facsimile machine, laser printer, direct digital platemaking machine, and the like. According to this electrophotographic method, the electrophotographic is charged, exposed to light, developed to form a toner image which is transferred on an image support (transfer paper, etc.), followed by fixation. Additionally, a cleaning process of the electrophotographic photoconductor may be performed.
As a conventional electrophotographic photoconductor used in the electrophotographic method, there are known, for example, an electrophotographic photoconductor provided with a photoconductive layer mainly comprising selenium or selenium alloy on an electroconductive support, an electrophotographic photoconductor comprising an inorganic photoconductive material such as zinc oxide•cadmium sulfide in a binder and an electrophotographic photoconductor using an amorphous silicone type material. However, recently, from the standpoint of achieving low cost, freedom of design of a photoconductor, and free of pollutant, an electrophotographic photoconductor made of organic material is widely used.
As the organic electrophotographic photoconductor, there are known, for example, an electrophotographic photoconductor using a photoconductive resin such as polyvinylcarbazole(PVK), an electrophotographic photoconductor using a charge transport complex type material such as PVK-TNF(2,4,7-trinitrofluorenone), an electrophotographic photoconductor using a pigment dispersed material such as phthalocyanine-binder, and a function-separation type electrophotographic photoconductor using a combination of a charge generation material and a charge transport material. Among them, the function-separation type electrophotographic photoconductor is becoming the focus of public attention.
The mechanism for forming electrostatic latent images using the function-separation type electrophotographic photoconductor is as follows. Firstly, the surface of the function-separation layered photoconductor is charged and thereafter exposed to light images. The light passes through the charge transport layer and is absorbed by a charge generation material for use in the charge generation layer. Upon absorbing light, the charge generation material produces a charge carrier. The charge carrier is injected into the charge transport layer and travels along an electric field generated by the charging step to neutralize the surface charge of the photoconductor. As a result, latent electrostatic images are formed on the surface of the photoconductor. There have been known and are currently used function-separation layered photoconductors employing a combination of a charge generation material which exhibits absorption within the UV region with a charge transport material which exhibits absorption mainly within the visible light region.
Most charge transport materials of the organic electrophotographic photoconductor developed for use in the electrophotographic method are low-molecular compounds. Since the low-molecular compounds alone cannot form a film, they are mixed with or dispersed in an inactive polymer.
However, the charge transport layer composed of a low-molecular charge transport material and inactive polymers is so flexible that there can be film abrasion due to the mechanical load to the photoconductor surface by a development system or cleaning system during repeated uses. As the film abrasion progresses, the electrostatic potential of the electrophotographic photoconductor is reduced, sensitivity is deteriorated, or image deterioration such as greasing and reduced image density by scratch on the electrophotographic photoconductor surface may occur. Further, in recent, due to minimization of the electrophotographic photoconductor as the electrophotographic apparatuses become faster and smaller, high durability of the electrophotographic photoconductor is an important issue.
In order to realize the high durability electrophotographic photoconductor, a protective layer is provided on the top surface layer of the photoconductor, and the protective layer is lubricated or cured, or a filler is added to the protective layer. In particular, the addition of the filler to protective layer is effective to enhance the durability of the electrophotographic photoconductor, improving wear resistance and mechanical durability. However, for a so-called electrophotographic method, electrical durabilities such as electrostatic potential or stable potential of a light exposure part as well as the mechanical durabilities due to repeated charging and light exposing processes are important. Though the mechanical durabilities are improved, reducing film abrasion, if the electrostatic potential is reduced or the potential of the light exposure parts is increased, sufficient electrostatic contrast cannot be obtained, causing deterioration of image quality.
Also, since there is a limit of charge movement in the charge transport layer, the electrophotographic process has difficulties in high speed operation and simplification. This is because the charge transport material of a low-molecular compound is used in a low content (usually 50 wt % or less). Thus, if the amount of the low-molecular charge transport material, the charge movement can be improved. However, this may impair film formability and wear resistance of the photoconductive layer.
As approaches to improve properties of the organic electrophotographic photoconductor, a technique ameliorating a binder resin of the organic photoconductor (for Example, Japanese Patent Laid-Open No. 5-216250) or techniques using a charge transporting polymer (for example, Japanese Patent Laid-Open No. 51-73888, Japanese Patent Laid-Open No. 54-8527, Japanese Patent Laid-Open No. 54-11737, Japanese Patent Laid-Open No. 56-150749, Japanese Patent Laid-Open No. 57-78402, Japanese Patent Laid-Open No. 63-285552, Japanese Patent Laid-Open No. 64-1728, Japanese Patent Laid-Open No. 64-13061, Japanese Patent Laid-Open No. 64-19049, Japanese Patent Laid-Open No. 3-50555, Japanese Patent Laid-Open No. 4-175337, Japanese Patent Laid-Open No. 4-225014, Japanese Patent Laid-Open No. 4-230767, Japanese Patent Laid-Open No. 5-232727, and Japanese Patent Laid-Open No. 5-310904) were disclosed.
However, the technique ameliorating the binder resin of the organic photoconductor has a problem in that significant improvement of wear resistance cannot be acquired due to compositional ration of low molecular charge transport material. The technique using the charge transporting polymer achieve success in improvement of wear resistance of film by employing high molecular material as the charge transport layer component. However, the photoconductor prepared by this technique is not sufficiently satisfactory as a permanent part without need for changing until the life span of a mother machine.
Meanwhile, the cleaning characteristics of the electrophotographic photoconductor are very important in terms of maintenance of the high image quality. This is because when impurities are adhered to the surface of the electrophotographic photoconductor surface, many image defects may occur, shortening the life span. Particularly, in case of the method for inhibiting the mechanical abrasion by adding a filler to the protective layer, it is necessary to have an excellent cleaning characteristics. Also, as the demand for high quality images increases recently, the size of toner particles used in the electrphotographic is smaller. When a smaller size toner is used, the cleaning characteristics of the electrophotographic photoconductor are worse. Further, in connection with a small size toner, spherical toners are studied. However, the spherical toners have cleaning characteristics poorer than the conventional crushed toners.
In Japanese Patent Laid-Open No. 07-295248, Japanese Patent Laid-Open No. 07-301936 and Japanese Patent Laid-Open No. 08-082940, it is disclosed a method of improving wear resistance of the photoconductor surface by adding fluorine-modified silicone oil to the surface layer to improve cleaning characteristics.
However, the fluorine-modified silicone oil tends to migrate to the surroundings of the surface during the formation of the surface layer and gathered then. As a result, the effects cannot last by the abrasion of the surface layer due to repeated uses.
Also, in order to improve wear resistance, various techniques to add finely-divided particle are attempted. For example, there are techniques of addition of silicone resin particles, fluorine-containing resin (Japanese Patent Laid-Open No. 63-65449), melamine resin particles (Japanese Patent Laid-Open No. 60-177349). Particularly, according to Japanese Patent Laid-Open No. 02-143257, polyethylene powders are added to the surface layer to reduce the frictional coefficient of the top surface and improve the cleaning characteristics, thereby improving the wear resistance of an electrophotographic photoconductor. According to Japanese Patent Laid-Open No. 02-144550, fluorine-containing resin powders are added to the surface layer to reduce the frictional coefficient of the top surface and improve the cleaning characteristics, thereby improving the wear resistance of an electrophotographic photoconductor. According to Japanese Patent Laid-Open No. 07-128872, Japanese Patent Laid-Open No. 10-254160, finely-divided particles of silicone are added to the surface layer to reduce the frictional coefficient of the top surface and improve the cleaning characteristics, thereby improving the wear resistance of an photoconductor
According to Japanese Patent Laid-Open No. 2000-010322 and U.S. Pat. No. 5,998,072, cross-liked organic particles are added to the surface layer to reduce the frictional coefficient of the top surface and improve the cleaning characteristics, thereby improving the wear resistance of an photoconductor. According to Japanese Patent Laid-Open No. 08-190213, finely-divided particles of a methylsiloxane resin are added to the surface layer to reduce the frictional coefficient of the top surface and improve the cleaning characteristics, thereby improving the wear resistance of an electrophotographic photoconductor. In these publications, the high durability was sought through reduction of frictional coefficient and surface energy at the surface of an electrophotographic photoconductor. However, these methods have the following problems.
That is, when resin powders or finely-divided particles are added to the surface layer of an electrophotographic photoconductor to improve the wear resistance of the surface of an photoconductive layer, the resin powders or particles have difficulties in being dispersed since they have a poor compatibility to the binder resin, generating defects which would be shown as black or white spots, whereby the residual potential increases during repeated uses. Also, the light transmission of the photoconductive layer may be impeded, and thus, there occurred problems such as reduction of sensitivity and charge transport performance and non-uniform image density.